/Engine/Plugins/Experimental/Mutable/Source/CustomizableObject/Classes/MuCO/CustomizableObject.h
These should typically match the name of the package, but with a different extension.
virtual void CookAdditionalFilesOverride ( const TCHAR * PackageFilename, const [ITargetPlatform](API\Developer\TargetPlatform\Interfaces\ITargetPlatform) * TargetPlatform, [TFunctionRef](API\Runtime\Core\GenericPlatform\TFunctionRef)< void(const TCHAR *Filename, void *[Data](API\Plugins\PCG\EPCGComponentDirtyFlag), int64Size)> WriteAdditionalFile )